Communication between two locations in an oil or gas well has been achieved using cables and optical fibers to transmit signals between the locations. In a petroleum well, it is, of course, highly undesirable and in practice difficult to use a cable along the tubing string either integral to the tubing string or spaced in the annulus between the tubing string and the casing. The use of a cable presents difficulties for well operators while assembling and inserting the tubing string into a borehole. Additionally, the cable is subjected to corrosion and heavy wear due to movement of the tubing string within the borehole. An example of a downhole communication system using a cable is shown in PCT/EP97/01621.
U.S. Pat. No. 4,839,644 describes a method and system for wireless two-way communications in a cased borehole having a tubing string. However, this system describes a communication scheme for coupling electromagnetic energy in a TEM mode using the annulus between the casing and the tubing. This inductive coupling requires a substantially nonconductive fluid such as crude oil in the annulus between the casing and the tubing. Therefore, the invention described in U.S. Pat. No. 4,839,644 has not been widely adopted as a practical scheme for downhole two-way communication.
Another system for downhole communication using mud pulse telemetry is described in U.S. Pat. Nos. 4,648,471 and 5,887,657. Although mud pulse telemetry can be successful at low data rates, it is of limited usefulness where high data rates are required or where it is undesirable to have complex, mud pulse telemetry equipment downhole. Other methods of communicating within a borehole are described in U.S. Pat. Nos. 4,468,665; 4,578,675; 4,739,325; 5,130,706; 5,467,083; 5,493,288; 5,576,703; 5,574,374; and 5,883,516.
PCT application, WO 93/26115 generally describes a communication system for a sub-sea pipeline installation. Importantly, each sub-sea facility, such as a wellhead, must have its own source of independent power. In the preferred embodiment, the power source is a battery pack for startup operations and a thermoelectric power generator for continued operations. For communications, ""115 applies an electromagnetic VLF or ELF signal to the pipe comprising a voltage level oscillating about a DC voltage level. FIGS. 18 and 19 and the accompanying text on pp. 40-42 does describe a crude system and method for getting downhole pressure and temperature measurements. However, the pressure and temperature sensors are passive (Bourdon and bimetallic strip) where movement varys a circuit to provide resonant frequencies related to temperature and pressure. A frequency sweep at the wellhead looks for resonant spikes indicative of pressure and temperature. The data at the well head is transmitted to the surface by cable or the ""115 pipeline communication system.
The use of predistorted signals for communicating along a nonlinear transmission path is well documented. U.S. Pat. No. 5,959,499 generally describes a method for determining a complex error signal and (during the operation mode of the system) generating a predistorted drive signal for a nonlinear transmission path. U.S. Pat. No. 5,963,090, generally describes a circuit applying non-linear characteristics to a signal. U.S. Pat. No. 5,251,328, generally describes compensating for amplitude distortion in the communications channel by predistorting the amplitude of transmitted signals. U.S. Pat. No. 4,291,277 generally describes compensating for distortion introduced into a multiamplitude signal format by predistorting the input signals before they are subjected to non-linearities. The degree of predistortion may vary and is updated to reflect changes in the system. U.S. Pat. No. 3,980,826 generally describes a method and means for transmitting a waveform of mixed frequency content over a transmission line to reduce the effects of transmitting digital signals over large distances.
Many of the wireless technologies attracting interest in the oil and gas industry have been derived from the commercial or defense industries. Since an oil well is embedded in a conducting formation and completion fluids are commonly filled with brine, it becomes difficult if not impossible to employ off-the-shelf, turnkey wireless systems in the subsurface environment. Whereas traditional cellular or line-of-sight terrestrial wireless systems enjoy the benefit of transmitting electromagnetic waves in a lossless medium (e.g., the air), subsurface wireless systems must necessarily transmit in a conductive and hence, lossy medium.
One characteristic of this lossy transmission medium is that electromagnetic signals are attenuated at a rate proportional to the square root of the transmitted frequency. This frequency-dependent attenuation is called the xe2x80x9cskin effectxe2x80x9d resistance or impedance of the transmission line. This skin effect attenuation causes the edges of square wave pulses or general pulses with fast rise times to be smoothed or xe2x80x9csmeared.xe2x80x9d As a result, it becomes increasingly difficult for the receiver detector to resolve and extract the information content of the original signal. Traditionally, to overcome this difficulty, either power must be increased or the data rate must be decreased so as to minimize the bit error rate and maximize the distance between transmitter-receiver pairs.
It would, therefore, be a significant advance in the operation of petroleum wells if an alternate means for compensating for the skin effect resistance of a lossy transmission line were provided. In cases where additional power is not available or the data rate cannot be reduced, an alternate means of compensating for the skin effect resistance is necessary to accurately maintain the integrity of signals transmitted along the line.
All references cited herein are incorporated by reference to the maximum extent allowable by law. To the extent a reference may not be fully incorporated herein, it is incorporated by reference for background purposes and indicative of the knowledge of one of ordinary skill in the art.
The problems created by the skin effect resistance of a lossy transmission line are solved by predistorting an input signal that will be transmitted along the line. By modeling the transmission line between a first location and a second location, an optimal predistortion can be applied to the input signal before the signal is transmitted from the first location on the transmission line. The predistortion allows an output signal received at the second location on the transmission line to be substantially similar to a desired target signal.
A communication system according to the present invention includes a pipe member with a first modem electrically connected to the pipe member at a first location. A second modem is electrically connected to the pipe member at a second location. A processor is provided for predistorting an input signal transmitted by the first modem to the pipe member. The second modem receives an output signal after the input signal is transmitted by the first modem.
A measurement system is provided for measuring an electrical formation characteristic for a petroleum well having a borehole and a piping structure that is located within the borehole. The measurement system includes a first modem electrically connected to the piping structure at a first location and a second modem electrically connected to the piping structure at a second location. The first modem imparts an input signal to the piping structure that is received as an output signal by the second modem. By observing the signal predistortion differences in the input signal and the output signal, the electrical formation characteristic of the well can be derived.
A method of communicating along a piping structure is also provided. The method is accomplished by predistorting an input signal and inputting the input signal onto the piping structure. The input signal is transmitted from a first location on the piping structure. An output signal is then received at a second location on the piping structure, the output signal being substantially similar to a desired target signal.